The Engram Revisited On the Elusive Permanence of Memory

This chapter describes memory as involving the endurance of physical changes in the organism and focuses particularly on the term engram—introduced by Richard Semon to refer to such changes. Although the systematic hunt for engrams using lesions in determining which parts of the brain impair the ability of animals to form and maintain memories proved futile before, the search was eventually revitalized and reasons were proposed as to why the previous experiments were in vain. Advanced methodologies were also recruited to the game, including localized brain stimulation, recording of nerve cell activity in the behaving animals, and functional brain imaging in humans. In analyzing the current transition in the interpretation of the engram, it is useful to spell out at the outset the two major, long-standing hypotheses in the neurobiology of memory. One is the “dual trace hypothesis” and the other the “consolidation hypothesis.”